1. Field of Invention
This invention relates to potentiometric measurements.
2. Prior Art
The removal of cyanides and other effluents from waste water, the cleansing of canned vegetables and poultry, and the purity of water used for industrial cooling and public consumption all depend on the use of chlorine additives, making the measurement of total residual chlorine of special interest. The measurement of residual chlorine is usually performed by colorimetry, ASTM D 1235-68; by amperometric titration, ASTM D 1253-76, D 1427; or by starch iodide titration, ASTM D 1253-76, D 1427.
Quick testing is a limiting requirement common to each of these three methods, and indeed, to any measurement of total residual chlorine. ASTM recommends that all tests be completed within 5 minutes of sampling, since chlorine demand of the sample may quickly diminish the residual chlorine present, resulting in determinations that are too low.
The colorimetric method measures total residual chlorine by reacting a sample, under acid conditions, with orthotolidine to form the yellow holoquinone of orthotolidine dihydrochloride. The color developed is approximately proportional to the amount of chlorine residual present and is quantitatively evaluated by comparison with standard colors.
The amperometric titration method prescribes a procedure for electrochemically determining the point at which a reducing agent reacts with all the total residual chlorine to eliminate all current. Total residual chlorine is calculated from the amount of reducing agent added to the sample. The titration endpoint is graphically determined on a plot of measured current versus volume of reducing agent.
The third method of measurement is starch iodide titration. This method involves the addition of iodide to the sample, reacting with all the total residual chlorine to form iodine, which in turn is reduced by titration with phenylarsene oxide solution. The iodine concentration is calculable from the volume of reagent added. The total residual chlorine concentration is then calculated from the relationship between iodine concentration and original total residual chlorine.
The starch iodide method includes any iodine present in the sample prior to the addition of iodide, introducing errors in total residual chlorine measurement. The inclusion of iodine present before testing for chlorine is of practical significance, however, since both chlorine and iodine are oxidizing halogens frequently monitored in waste water.
The colorimetric method is ineffective in both colored and dirty water, yielding results with poor precision and accuracy, thus placing a severe limitation on its widespread use. Several other limitations arise in the use of the amperometric and starch iodide titration methods. Since both require accurate measurement of volumes and precise use of laboratory equipment in a short time span, trained technicians are necessary for accurate results. In the amperometric method, the titration endpoint is interpreted from the current -- volume curve, again calling for special expertise in performing the test in a repeatable fashion. There is also the inconvenience of a time delay, since data manipulation precedes the test results.